In Nebraska, along the Platte River, it’s uranium. In Maine, New Hampshire and Massachusetts, it’s arsenic. In California, boron. And in the Texas Panhandle, lithium.

Throughout the nation, metals and other elements are tainting private drinking water wells at concentrations that pose a health concern.

For one element – manganese – contamination is so widespread that water wells with excessive levels are found in all but just a few states. Arsenic, too, is a national problem, scattered in every region.

In the first national effort to monitor wells for two dozen trace elements, geologists have discovered that 13 percent of untreated drinking water contains at least one element at a concentration that exceeds federal health regulations or guidelines. That rate far outpaces other contaminants in well water, including industrial chemicals and pesticides.

For public wells, the discovery is less of a concern, since water suppliers regularly test for contaminants and remove them to comply with federal standards. The most troubling finding involves the widespread contamination of private wells, which are unmonitored and unregulated.

“It was a bit surprising how many of these trace elements had exceedances of human health benchmarks, especially compared to other contaminants we are often concerned about,” said Joseph Ayotte, a hydrologist with the U.S. Geological Survey, which conducted the research. “The findings certainly underscore the message we hear from the public health agencies, that everyone should test their wells for a suite of trace elements.”

An estimated 15 million U.S. households – about 60 million people – regularly depend on private water wells, according to the Centers for Disease Control and Prevention. Most private well owners occasionally test for bacteria, but rarely, if ever, test for anything else.

The importance of ground water

Nearly half of all drinking water in the country comes from ground water, and usage is increasing worldwide as freshwater supplies from rivers are running low and encumbered by bitter feuds.

“Ground water is very important. It’s the invisible link to our water supply that people don’t really think about,” said David Wunsch, director of science and technology at the National Ground Water Assn. “It’s underground – out of sight, out of mind – and that’s exactly why we’ve come across the pollution problems we have.”

Testing wells for contaminates

The geologists tested more than 5,000 wells in 40 aquifers for 23 elements – all metals and metal-like substances – plus radon.

Drier regions of the country, mostly in the West, had more wells with excessive trace elements than humid regions, and urban areas had more than agricultural areas.

“Wells with human health benchmark exceedances were widespread across the United States; they occurred in all aquifer groups and in both humid and dry regions,” the report says.

“Trace elements are a widespread chronic health problem,” Wunsch said. “It’s not like something that’s really dramatic where people will get sick right away, but the public needs to be informed more. Some of the things listed here – arsenic, barium, lead, cadmium, for example – can be toxic in small quantities.”

Wunsch said people usually test their well water only when it tastes funny or smells bad. “Private well water is not regulated like the public water supply is. So it’s up to the homeowner to take care of it himself,” he said.

Mary McClintock has been drinking well water since she bought her home in Conway, Mass., 27 years ago. “I remember having the well tested before buying the house, and perhaps at one other time early on,” she said.

McClintock said she has no health concerns about her water, although a friend who lives in Leverett 15 miles away discovered his well water had excessive arsenic after he tested it at the recommendation of state officials. He and his family now drink bottled water.

“Why haven’t I tested my well? No particular reason except assuming it is a deep well and not in danger of contamination,” she said.

Deeper wells aren't necessarily safer

The USGS maps show that arsenic is mostly a problem in parts of New England east of where McClintock lives. But geologists say that it's a misconception to believe that deeper wells are safer. Elements can be found at any depth, and some, like arsenic, are even worse in deeper wells.

“We actually anticipate that deeper wells should have more arsenic than shallow ones based on the geochemistry of mobilization of arsenic. But high concentrations can occur at any depth if the geochemical conditions are right,” Ayotte said. Sand and gravel soils are most conducive to contamination, which also depends on factors such as pH and climate.

Hot spots for contamination

Overall, 19 percent of the 5,183 untreated public, private and monitoring wells exceeded the health-based levels. When private drinking water wells were separated from the pack, 13 percent exceeded the health standards or guidelines.

But that national average is a bit deceptive, since some regions have a far greater frequency of problems. Hot spots for elements are congregated in clusters.

Eastern New England, for example, has a cluster of high arsenic concentrations. “What we did not know was how to connect the dots in New England states,” Ayotte said. “It became apparent [in the new study] that the high arsenic values formed a contiguous belt between three states, Maine, New Hampshire and Massachusetts.”

They also found that manganese is “widespread but more prevalent in the East. It’s similar with arsenic, you can find it almost anywhere at high levels,” said Ayotte, lead author of the USGS study, also authored by Jo Ann Gronberg and Lori Apodaca.

Other arsenic hot spots include the Sacramento and Los Angeles regions of California, western Nevada, the Phoenix area, the Texas Panhandle and New Mexico along the Rio Grande, according to the new study’s maps. Others are found in Illinois, Ohio, south Florida and New Jersey.

“Arsenic is definitely a national problem with a local flavor,” Ayotte said.

Most elements found in ground water are derived from natural sources, since they are part of the Earth’s crust. But even though the source is overwhelmingly natural, “the conditions that make it go into the water are not always natural,” Ayotte said. Elements can contaminate water from farms, urban runoff, mining and factories, either directly or by altering aquifer conditions to mobilize them in rocks and soil.

Drinking water standards

The EPA has set enforceable drinking water standards for 11 trace elements, what Wunsch called “the really nasty ones.” But others, including manganese, have only USGS guidelines based on a small number of health studies.

Private wells are not subject to federal standards because there are tens of millions of them, which would make it unenforceable.

Instead, EPA officials said in response to the new findings that people should test their private wells “if they suspect possible contamination.” The new USGS maps are useful for well owners who want to check the hot spots.

Testing a well for arsenic costs as little as $15 to $30, while treatment systems for removing arsenic cost $1,200 to $3,000, according to the Massachusetts Dept. of Environmental Protection.

The report notes that elements “far outpace” other pollutants, many of which get far more public attention. The 19 percent compares with 7 percent for nitrates and 1 to 2 percent for pesticides and volatile organic compounds, based on previous USGS research.

“We often get more upset about these anthropogenic contaminants but we have to remember that these naturally occurring elements are oftentimes more of a widespread problem,” Ayotte said. “Not to diminish the importance of the others, but trace elements are also hugely important and arguably more so.”